A known composition having a continuous phase and a disperse phase dispersed finely in the continuous phase is an emulsion having a continuous phase liquid and a disperse phase liquid dispersed finely in the continuous phase liquid or a microbubble composition having a continuous phase liquid and a disperse phase gas dispersed finely in the continuous phase liquid. A conventional emulsion has been produced by addition of a liquid which serves as a disperse phase and an emulsifier (e.g., a surfactant) to a liquid which serves as a continuous phase in order to form a mixed liquid which is then stirred mechanically to make the disperse phase finer. A known concrete means for the mechanical blending is stirrer, homogenizer, or colloid mill. Another method which has been used is a method of making a disperse phase finer by ultrasonic irradiation to a mixed liquid to thereby generate cavitation.
However, in emulsions produced by these methods, the size of their disperse phase particles was not uniform. In general, the solubility of a disperse phase liquid in a continuous phase liquid depends on the surface curvature of the disperse phase and is represented by the Thompson-Freundlich equation shown below.RTIn(Ca/C∞)=(2γ/a)·(M/ρ)
wherein a represents the radius of disperse phase particles, Ca represents the solubility of the disperse phase particles having radius a, C∞ represents the solubility of the disperse phase liquid in the vicinity of a flat interface, γ represents interfacial tension, M represents the molecular weight of the disperse phase liquid, ρ represents the density of the disperse phase liquid, R represents gas constant, and T represents absolute temperature.
More specifically, the equation shows the following points: the smaller the size of disperse phase particles is, the higher the solubility of the disperse phase in a continuous phase is; and the larger the size of disperse phase particles is, the lower the solubility of the disperse phase in a continuous phase is. Thus, if the size of disperse phase particles is not uniform, when the disperse phase molecules dissolved in a continuous phase (dissolved molecules) exist in the vicinity of smaller disperse phase particles, the dissolved molecules diffuse and move within the continuous phase and are absorbed by larger disperse phase particles. As a result, smaller disperse phase particles gradually become smaller while larger disperse phase particles become larger. This phenomenon to cause the destabilization of an emulsion is also called Ostwald ripening. If the sizes of disperse phase particles can be uniformly controlled, the solubility of the individual disperse phase particles in a continuous phase becomes almost equivalent, and this can prevent the destabilization of an emulsion due to Ostwald ripening.
In addition, an emulsion is broadly used in foods, cosmetics, chemical products, and pharmaceutical products, and the size of its disperse phase particles needs to be changed depending on intended uses. However, it was difficult to control the particle size accurately in conventional production methods. In particular, an emulsion containing polymerizable monomers in its disperse phase is important as a raw material for a suspension polymerization method by which a disperse phase is directly polymerized in the presence of a polymerization initiator. Concerning a suspension polymerization method, there is a particularly strong demand for polymer fine particles having an intended, uniform size, but it was difficult for conventional technology of producing emulsions to satisfy the demand.
In order to solve the above problems, a method of producing an emulsion by pressing a disperse phase into a liquid to serve as a continuous phase via a porous membrane having uniform micropores has been suggested (Patent Document 1: JP 2003-270849A; Patent Document 2: JP H2-95433A). The patent documents each show that the membrane emulsification method enables a disperse phase having an intended and almost uniform particle size to be obtained by the choice of the pore size of the porous membrane. For example, the method disclosed in Patent Document 1 produces a toner having an average particle size of 4 to 15 μm and a polydispersity index D25/D75 of 1.02 to 1.40 (Paragraph [0028]). In the method disclosed in Patent Document 1 or 2, a cylindrical body the circumferential surface of which is formed by a porous membrane is used and a continuous phase liquid is flowed into the cylindrical body in parallel to its axis (hereinafter also referred to as “axial-flow system”). The flow speed of the continuous phase liquid is approximately 0.5 to 2 m/s (for example, Patent Document 2, page 196, upper left column, line 11). According to Paragraph [0024] of Patent Document 1, the volume of the disperse phase liquid to be treated per unit time in the method (hereinafter also referred to as “supply speed (m3/m2·h)”) is 50 to 1000 ml/min per 1 m2 of a membrane area (3 to 60×10−3 [m3/m2·h]). That is, in the axial-flow system, the supply speed of the disperse phase liquid was as low as 10−3 order level, which was not sufficient for practical use. Another problem concerning the axial-flow system was that a higher supply speed than this level prevented the production of disperse phase particles having a uniform size; that is, the problem was that the polydispersity increased.
Meanwhile, Non-Patent Document 1 or 2 suggests a method of producing an emulsion efficiently by increasing the supply speed of a disperse phase liquid. Specifically, Non-Patent Document 1 or 2 discloses a method of producing an emulsion characterized by use of an asymmetrically structured porous glass membrane and the supply speed of the disperse phase liquid which is increased to 10−2 order level (Non-Patent Document 1: Journal of Membrane Science, Vol. 299 (2007), 190-199; Non-Patent Document 2: Masato Kukizaki, Miyazaki Prefecture Industrial Technology Center, “Preparation of Porous Glass Membrane and Its Application to Monodisperse Emulsions and Nanobubbles” (“Takoushitsu garasumaku no Chousei to Tanbunsan Emarujyon oyobi Nanobaburu heno Ouyou”), a material for the first lecture for promotion of the cooperation between industry and agriculture). If a porous membrane is likely to get wet with a disperse phase liquid, the particles of the disperse phase generated on the surface of the porous membrane are unlikely to be released from the membrane and thus the supply speed of the disperse phase liquid cannot be increased. However, Non-Patent Document 1 or 2 shows that since the disclosed method uses the asymmetrically structured porous glass membrane, a continuous phase liquid can pass oppositely through the membrane; thus, the membrane is unlikely to get wet with the disperse phase liquid and the particles of the disperse phase are likely to be released from the porous membrane, and this increased the supply speed of the disperse phase liquid.